An electrically conductive transparent film has conventionally been formed on an electrical insulating substrate either the following process (1) or (2):
(1) a dry process such as PVD (including sputtering and ion plating) or CVD to form a conductive transparent film of a metal oxide type, e.g., antimony-tin mixed oxide or tin-indium mixed oxide on a substrate; PA1 (2) a wet process using a conductive coating composition comprising an electrically conductive powder, e.g., one of the above-described mixed oxides, and a binder.
The dry process (1) produces a film having both good transparency and good conductivity. However, it requires a complicated apparatus having a vacuum system and has poor productivity. Another problem of the dry process is that it is difficult to apply to a continuous or big substrate such as photographic films or show windows.
On the other hand, the wet process (2) is advantageous in that it requires a relatively simple apparatus, has high productivity, and is easy to apply to a continuous or big substrate.
The conductive powder used in the wet process is a very fine powder having an average primary particle diameter of 0.5 .mu.m or less so as not to interfere with the transparency of the resulting film. However, such a fine powder is strong in binding power between particles and tends to re-agglomerate in a coating composition, thereby causing the coating composition to have a low stability.
Furthermore, since electric conductivity results from direct contact between powder particles, it is necessary for the conductive powder to be present in a dry film in a large proportion on the order of more than 50% by weight in order to provide the film with adequate electric conductivity. This increases the costs of a conductive film made by the wet process and leads to losses of physical (mechanical and thermal) properties of the film.
Carbon fibers, particularly graphitized carbon fibers have good electric conductivity and it has been attempted to use carbon fibers as a conductive material. In particular, those carbon fibers prepared by the vapor-phase growth (pyrolysis) method and optionally subjected to graphitization by heat treatment are attractive since they are hollow or solid thin fibers having an outer diameter of 0.1-10 .mu.m and having high electric conductivity.
Recently, carbon fibers much thinner than the above-described fibers by the conventional vapor-phase growth method have been developed. See, Japanese Patent Kokoku Publications Nos. 3-64606 and 3-77288; Japanese Patent Kokai Applications Nos. 1-131251, 3-174018, and 5-125619; and U.S. Pat. Nos. 4,663,230; 5,165,909; and 5,171,560. Such very thin carbon fibers (hereinafter referred to as carbon microfibers) are hollow fibers having a hollow core and their outer diameter is in the range of several to several ten nanometers (nm). Due to the diameter on the order of nanometers, carbon microfibers are also called nanotubes or carbon fibrils. It has been proposed to use such fibers as a reinforcing material in the manufacture of composite materials, or to utilize their electrical conductivity and employ them in antistatic materials, electrodes, and shields against electromagnetic waves.
U.S. Pat. No. 5,098,771 describes an electrically conductive coating and ink containing hollow carbon microfibers. In the coating disclosed in that patent, in order to give the coating electrical conductivity, the hollow microfibers are employed in an amount of 1-4 wt % with respect to a binder resin.
An object of the present invention is to provide a transparent electrically conductive film which does not have the drawbacks of the above-described wet-process (2) and a coating composition for forming this transparent electrically conductive film.
More particularly, it is an object of the present invention to provide a transparent electrically conductive film having a reduced content of electrically conductive materials and a coating composition for forming the film.
Another object of the present invention is to provide an electrically conductive film which contains carbon fibers as an electrically conductive material while maintaining its transparency as well as a coating composition for forming the film.
Yet another object of the present invention is to provide a novel, transparent antistatic layer.